Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating preforms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Laboratories, Inc., has been found most useful because the process enables large scale production of preforms which yield very low loss optical fiber.
During the fabrication of preforms by the MCVD process, precursor, reactant-containing gases, such as SiCl.sub.4 and GeCl.sub.4 are passed through a rotating substrate tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of glass to provide a preform tube. Once a sufficient number of layers have been deposited, the preform tube is then heated to cause it to be collapsed to yield a preform or preform rod as it is often called.
Increased demand for optical fiber has prompted efforts to increase the productivity of the MCVD process. However, the MCVD process rate is limited by several factors including the thickness of the wall of the substrate tube. To obtain optical fiber having optimal optical and geometrical characteristics, the preform must have a core-to-cladding geometric ratio within specified limits. Increasing the mass of the substrate tube to obtain a larger preform requires that the wall of the substrate tube be made thicker. Increasing the thickness of the wall of the substrate tube, however, reduces the rate of heat transfer to the reactant-containing gases, thereby increasing the time required to deposit and sinter each layer of glass particulates. If the wall of the substrate tube is too thick, insufficient heat transfer may occur, which may result in the formation of bubbles or incomplete sintering.
One way in which the productivity of the MCVD process can be increased is first to produce a preform having a relatively large core and a larger than desired core-to-cladding geometric ratio. This preform is inserted into a glass tube which is referred to as an overcladding tube and which is then collapsed onto the preform. This is referred to as the rod and tube technique. It is desirable that any added eccentricity of material about the preform core due to overcladding should be minimized. Radial misalignment between the overcladding tube and the large core preform also should be minimized, otherwise the resultant drawn fiber core may be too eccentric which inhibits proper splicing of the drawn fiber to another. This may be difficult to do inasmuch as for relatively thick preforms, the overcladding is accomplished in stages. Each successive tube has its own longitudinal centroidal axis as does the core, and the multiple overcladding increases the probability for eccentricity among multiple tubes.
In another process for increasing production rates, soot overcladding is used to provide an enlarged preform. In such a process, soot is deposited onto a substrate such as, for example, a preform rod which may be manufactured by the modified vapor chemical deposition process. See, for example, priorly identified U.S. Pat. No. 4,217,027. After the soot has been deposited to provide a boule, the soot is sintered by subjecting successive increments of length of the boule to a heat source. Typically, the boule and rod are suspended with their longitudinal axes, which are coaxial, being vertical and moved downwardly into a furnace. Of course, the preform rod provided by the modified chemical vapor deposition process is modified to provide a suitable core-to-cladding ratio.
Soot overcladding has certain advantages over the rod and tube process. For example, as mentioned hereinbefore, as larger tubes or as multiple tube clads are used, the eccentricity of the core and cladding generally increases. This is not true as the size of the soot boule increases. However, other problems relating to soot boules must be overcome.
Problems have arisen in the sintering of enlarged boules. Typically, a source applies heating energy to the outer surface of the boule which causes the outer layer of soot to consolidate and become a layer of transparent solid glass. When soot is deposited onto the substrate rod, the silicon tetrachloride and oxygen reactants provide silicon dioxide, which is the soot, and chlorine as a byproduct. When the soot is deposited on the substrate rod, it is loosely bound with voids between particles on the order of 1 .mu.m being filled with the chlorine and other by-product gases. Subsequently, some or all of the trapped gases are replaced with helium, during sintering.
In order to provide a preform from which suitable optical fiber is drawn, the helium gas and any chlorine gas remaining in the voids must be driven out. If the heating proceeds inwardly toward the substrate rod, this may be difficult to do inasmuch as the gases may become trapped between layers of glass and the substrate rod. As the boule is moved into a furnace, for example, a sintering wavefront extends from a position adjacent to the substrate rod to an outer surface of the boule. This causes the gases to travel along the rod toward the upper end thereof to escape. With such a lengthened path of escape for the gases, the process is less efficient than desired.
Also, and perhaps more importantly, a boule having an extremely large outer diameter may not be processable by currently used methods. After the heat energy causes a layer of soot adjacent to the outer surface of the boule to be consolidated into glass, the heat energy is required to penetrate that layer to cause the next successive inner stratum of soot to consolidate. As a result, an inner stratum of soot may not be consolidated if the boule is too large. As should be apparent, this possibility places disadvantageously a limitation on the boule size.
What is needed and what seemingly is not provided by the prior art are methods of soot overcladding in which relatively large boules may be provided. Such methods must be competitive costwise with current methods and must provide a boule which is sintered throughout such that quality optical fiber may be drawn therefrom.